Field of the Invention
The present invention concerns a method and an apparatus for implementing a simultaneous multislice (SMS) magnetic resonance (MR) data acquisition, and in particular to an SMS data acquisition protocol making use of a turbo spin echo-based (TSE-based) reference scan.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field (called the B0 field), by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The acquired MR signals are digitized and entered into an electronic memory, organized as k-space, as k-space data. The k-space data are also referred to as raw MR data. The k-space memory has a number of individual locations that are available for entering the digitized signal thereat (data entry points), and the path of data entry points along which the digitized data are entered is called the k-space trajectory. The acquired data can be entered into k-space linearly (line-by-line of data entry points) or radially, along a straight or curved path proceeding from the center of k-space toward the periphery of k-space.
Many reconstruction algorithms are known for operating on the k-space data to convert the k-space data into image data representing an image of the volume of the examination object from which the raw MR data were acquired. This reconstruction algorithm is executed in an image reconstruction computer, resulting in an image data file from the computer that can be shown at the display screen of a monitor, or archived for storage.
After the nuclear spins have been flipped by the RF excitation pulse, the resulting MR signal exhibits an exponential decay in strength as the excited nuclear spins relax. This decaying signal is referred to as an echo signal, or simply as an echo. A commonly used data acquisition sequence of appropriately timed RF excitations and gradient pulse activations (switchings) is the echo planar imaging (EPI) sequence. In an EPI sequence, instead of measuring only one echo after each excitation pulse, multiple echoes are detected by multiple activations of the readout gradient after a single excitation pulse. In an EPI sequence, therefore, the total echo time of the decaying MR signal that follows the excitation of the nuclear spins is divided into a number of individual echo times, corresponding to the number of activated readout gradients.
The turbo spin echo (TSE) sequence is the “clinical workhorse” sequence for MR imaging, by virtue of being the most utilized sequence for all types of body region imaging. A TSE sequence has several echo trains, and in each echo train, multiple phase encoding lines of the entirety of k-space are scanned (filled with data) after one excitation pulse. This is achieved by refocusing the spins after each readout line, utilizing refocusing RF pulses. Compared to a conventional spin echo (SE) sequence, the acquisition time in a TSE sequence is reduced by the number of refocused echoes in one echo train. This reduction is known as the turbo factor. A TSE sequence diagram showing one echo is depicted in FIG. 1.
A technique for accelerating MR data acquisition, and thereby reducing the time that the patient must spend within the MR scanner, is the simultaneous multi-slice (SMS) technique. Details of techniques for SMS data acquisition can be found in Setsompop et al. “Improving Diffusion MRI Using Simultaneous Multi-Slice Echo Planar Imaging,” Neurolmage, Vol., 63, pages 569-580 (2012) and Breuer et al. “Controlled Aliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA) For Multi-Slice Imaging,” Magnetic Resonance in Medicine, Vol. 53, pp 584-591 (2005) and Larkman et al. “Use of Multi-Coil Arrays for Separation of Signal from Multiple Slices Simultaneously Excited,” Journal of Magnetic Resonance Imaging, Vol. 13, pp 313-317 (2001) and Souza et al., “SIMA: Simultaneous Multislice Acquisition of MR Images by Hadmard-Encoded Excitation,” Journal of Computer Assisted Tomography, Vol. 12(6), pp 1026-1033 (1988).
In general, the SMS technique involves the simultaneous excitation of a number of slices of a volume that is to be imaged, and the simultaneous acquisition of the resulting MR signals from the multiple slices. The number of slices that are simultaneously excited are typically grouped in a slice group. The SMS technique is typically characterized by an acceleration factor that indicates how many slices are simultaneously excited in a slice group by one individual radio-frequency pulse. The acceleration factor is at least two.
Since the k-space data for all of the simultaneously acquired slices are entered simultaneously into k-space, the subsequent reconstruction of the respective images of the individual slices involves a separation of the k-space data for the multiple slices. The data for the respective slices can be separated using a known slice separation method, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, noted in the aforementioned article by Setsompop et al. This enables the SMS technique to be used to acquire a number of slices at the same time. A schematic illustration of this known technique is shown in FIG. 2, for the example of three slices S1, S2 and S3.
As noted above, in SMS imaging, multiple slices are excited at the same time. In the example of FIG. 2, this takes place using blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration), and is described in the article by Setsompop et al. “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multi-Slice Echo Planar Imaging with Reduced g-factor Penalty,” Magnetic Resonance in Medicine, Vol. 67(5), pp. 1210-1224 (2012) (shortened to blipped CAIPI therein).
In SMS imaging, in order to excite multiple slices simultaneously, a multi-band (MB) excitation pulse is used. For each slice in which the nuclear spins are to be excited, a linear phase ramp is added to a standard excitation pulse. The linear phase results in a slice shift in the spatial domain. The pulse shapes for all bands are added (superimposed), resulting in a baseband-modulated MB pulse.
The individual signals that are superimposed as a result of the MB pulse are received from respective individual coils of the MR data acquisition scanner. These coils necessarily respectively occupy different spatial positions, but are in close enough proximity to each other so that the same nuclear spins will be detected by more than one of the multiple coils. Because each coil is situated at a different position in space, however, the effect of each individual coil on the reception of a given individual nuclear spin will be slightly different, and must be taken into account. This is done by calculating a so-called g-factor (geometry factor) for the coil array that is used. The extent to which the g-factor degrades the resulting reconstructed image is called the g-factor penalty. In order to reduce the g-factor penalty in SMS imaging, interslice image shifts are deliberately induced during the readout in blipped CAIPI, either by gradient blips on the slice axis or by modulating the phase of the RF pulses. After the data have been acquired, the simultaneously excited slices are collapsed into a single slice for entry of the data into k-space. The individual slices can be separated in the post-processing, utilizing the aforementioned slice GRAPPA technique, as schematically illustrated in FIG. 2.
It is known to combine SMS and TSE techniques. In order to allow for the unfolding of the collapsed multi-band data that occurs in such a combination, a single-band reference scan, which covers all slices from which data are to be acquired, must be used to acquire reference data, in addition to acquiring the multi-band data. The current SMS TSE implementation uses a gradient echo (GRE) reference scan preceding the SMS data acquisition. Although this scan is very fast, it has been found to be prone to slice crosstalk and fat ring ghosting artifacts, which limit its application in routine clinical practice. A suspected cause of these drawbacks is the presence of inconsistencies between the GRE reference scan and the TSE imaging scan. Such inconsistencies might arise due to the presence of B0 inhomogeneities, different fat-water shift behavior, and/or different slice profiles.